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Abstract:

The invention relates to a microbattery that comprises a stack on a
substrate, covered by an encapsulation layer and comprising first and
second current collector/electrode assemblies, a solid electrolyte and
electrical connections of the second current collector/electrode assembly
to an external electrical load. The electrical connections are formed by
at least two electrically conductive barriers passing through the
encapsulation layer from an inner surface to an outer surface of the
encapsulation layer. Each of the barriers has a lower wall in direct
contact with a front surface of the second current collector/electrode
assembly and an upper wall opening onto the outer surface of the
encapsulation layer. The barriers form a compartmentalization network
within the encapsulation layer.

Claims:

1. A microbattery that comprises a stack of solid thin layers formed on a
substrate, said stack being covered by an encapsulation layer and
successively comprising: a first current collector/electrode assembly in
direct contact with the substrate, a solid electrolyte, a second current
collector/electrode assembly having a front surface in direct contact
with the inner surface of the encapsulation layer and a rears surface in
direct contact with the solid electrolyte and, electrical connections of
the second current collector/electrode assembly to an external electrical
load, wherein the electrical connections are formed by at least two
electrically conductive barriers passing through the encapsulation layer
from the inner surface to the outer surface of said encapsulation layer,
each of said barriers having a lower wall in direct contact with the
front surface of the second current collector/electrode assembly and an
upper wall opening onto the outer surface of the encapsulation layer and
in that said barriers form a compartmentalization network within the
encapsulation layer.

2. The microbattery according to claim 1, wherein at least two barriers
are adjacent and parallel to one another and are separated by a part of
the encapsulation layer.

3. The microbattery according to claim 2, wherein the distance separating
the two adjacent and parallel barriers is smaller than or equal to the
inverse of the square root of the dislocation density of said
encapsulation layer.

4. The microbattery according to claim 1, wherein the
compartmentalization network has a meshed structure with patterns in the
form of a parallelogram, preferably in the form of a grid.

5. The microbattery according to claim 1, wherein the side walls of each
barrier are perpendicular to the front surface of the second current
collector/electrode assembly.

6. The microbattery according to claim 1, wherein the upper wall of the
barrier forms a common flat surface with the outer surface of the
encapsulation layer.

7. A manufacturing method of a microbattery according to claim 1,
comprising the following successive steps: full-wafer deposition, on the
substrate of a stack of thin layers successively forming a first current
collector/electrode assembly, a solid electrolyte, and a second current
collector/electrode assembly, full-wafer deposition of a first protective
layer on the second current collector/electrode assembly, making in the
first protective layer at least one trench that forms a matrix to form
the barriers, said trench passing through the thickness of the first
protective layer and opening onto the front surface of the second current
collector/electrode assembly and, filling the trench with an electrically
conductive material.

8. The method according to claim 7, wherein the filling step comprises
deposition of a thin layer of electrically conductive material on the
first protective layer followed by selective etching of said thin layer.

9. The method according to claim 7, comprising one or more etching steps
of the stack to localize the microbattery on the substrate, after filing
of the trench.

10. The method according to claim 9, wherein, after etching of the stack,
a second protective layer is made covering all the lateral parts of the
stack and forming an encapsulation layer with the first protective layer.

Description:

BACKGROUND OF THE INVENTION

[0001] The invention relates to a microbattery comprising a stack of solid
thin layers formed on a substrate, said stack being covered by an
encapsulation layer and successively comprising: [0002] a first current
collector/electrode assembly in direct contact with the substrate, [0003]
a solid electrolyte, [0004] a second current collector/electrode assembly
having a front surface in direct contact with the inner surface of the
encapsulation layer and a rear surface in direct contact with the solid
electrolyte and, [0005] electrical connections of the second current
collector/electrode assembly to an external electrical load.

[0006] The invention also relates to a manufacturing method of such a
microbattery.

STATE OF THE ART

[0007] A microbattery is defined as being an all-solid electrochemical
generator formed by an active stack of thin layers that constitute the
(positive and negative) electrodes separated by an electrolyte. Such a
microbattery further comprises metal current collectors. All the layers
of the microbattery are in the form of thin films obtained by PVD
(Physical Vapor Deposition) or CVD (Chemical Vapor Deposition).

[0008] The positive electrode is generally a lithium insertion material,
for example lithiated metal oxides, and the electrolyte is an electronic
insulator with a high ionic conductivity.

[0009] For lithium microbatteries, three main categories of microbattery
implementing Li.sup.+ ions can be distinguished depending on the nature
of the negative electrode: microbatteries called "lithium-metal",
"lithium-ion" or "lithium-free" microbatteries.

[0010] Microbatteries called "lithium-metal" microbatteries present the
best electrochemical properties, in particular in terms of potential and
stability of the charging and discharging capacity. This first category
of lithium micro-battery comprises a negative electrode made from
metallic lithium.

[0011] Microbatteries called "lithium-ion" (or Li-ion) microbatteries have
a negative electrode formed by a lithium intercalation or insertion
material. The Li.sup.+ cations go back and forth between the negative and
positive electrodes on each charging and discharging of the microbattery.
This category of micro-battery enables the usual microfabrication
techniques to be used, but generally presents less good electrochemical
performances on cycling.

[0012] Finally, microbatteries called "lithium-free" microbatteries
comprise a metallic current collector as negative electrode. During a
first charge, the Li.sup.+ ions migrate from the positive electrode and
are blocked by the current collector. The Li.sup.+ ions form a layer of
metallic lithium on the current collector by electro-deposition. The
battery then behaves in the same manner as a lithium-metal battery, with
a small capacity drop during the first cycle.

[0013] Lithium microbatteries are particularly interesting due to their
high mass density, their high useful energy storage surface and their low
toxicity. They are however sensitive to air and in particular to
moisture. In order to protect the active stack of the lithium
microbattery, a packaging is generally made in order to isolate the stack
from outside contact thereby preventing any contamination from the
environment.

[0014] Different microbattery architectures exist, in particular
architectures that present a planar or vertical electrical connection,
i.e. in which the final electrical connection of the microbattery to an
external load is made either on the same plane as that of the current
collector or on a plane that is vertical with respect to the latter.

[0015] For example purposes, the document WO2008/011061 describes a
micro-battery with a planar electrical connection. As represented in FIG.
1, the microbattery typically comprises a stack 1 on a substrate 2. Stack
1 is successively formed by a first current collector 3, a first
electrode 4 covered by a solid electrolyte 5, a second electrode 6 and a
second current collector 7. Stack 1 is encapsulated by a coating 8 that
is inert to the elements forming stack 1 and designed to tightly seal the
latter and to protect it against external contamination. A contact
connection is made through a metal layer 9 on coating 8 and coupled to
the second current collector 7 via a through hole 10 made in coating 8.
The contact connection connects second current collector 7 situated above
stack 1 and first current collector 3. Through hole 10 passes through
coating 8 and defines a localized area 11 of stack 1 covered only by
metal layer 9 forming the contact connection. Localized area 11 then
constitutes a fragile point of the microbattery that is more sensitive to
mechanical stresses. In particular, operation of the lithium microbattery
being based on transport of current by the lithium ions, when charging
and discharging of the microbattery take place, the electrodes undergo
deformations due to insertion and extraction, also called de-insertion,
of the lithium ions in the electrodes. These repeated modifications of
volume rapidly give rise to mechanical damage. The parts of layer 9
comprised between localized areas 11 and the fixed connecting point with
layer 3 are more sensitive to mechanical stresses which can consequently
give rise to failures in use and be the cause of a decrease in the
performances of the micro-battery.

[0016] As represented in FIG. 1, electrical connection of this type of
architecture with an external electrical load (not shown) is performed
conventionally by contact pads 12 arranged on each side of coating 8, in
the extension of current collector 3. Contact pads 12 form contact
terminals which enable the microbattery to be connected for example to
the external electrical load, to one or more additional microbatteries,
to an electronic chip or to any electrical load. Final connection to the
external load is typically made by a soldering step with a wire. In
addition to a complex and delicate implementation, this type of
architecture presents a drawback in terms of integration. A part of the
surface of first current collector 3 is in fact occupied by contact pads
12, which consequently limit the surface dedicated to first electrode 4.
The electrochemical properties of the microbattery, and for example the
capacity (in μAh.cm-2), are however proportional to the active
surface of the microbattery, in particular to the dimensions of the first
electrode 4/electrolyte 5 and first electrode 4/first current collector 3
interfaces. A loss of active surface is then observed that is synonymous
with a loss both of electrochemical performance and of integration
density (number of devices per surface unit on the substrate).

[0017] For example purposes, the document US-A-20070238019 describes a
microbattery with a vertical electrical connection with respect to the
main plane of substrate 2. In this configuration represented in FIG. 2, a
stack deposited on a substrate 2 conventionally comprises a cathode 4 and
an anode 6 separated by an electrolyte 5. Substrate 2 has a front surface
13 and a rear surface 14. First and second current collectors,
respectively 3 and 4, are arranged on front surface 13 of substrate 2.
Substrate 2 presents pass-through connections 15 from current collectors,
3 and 4, on the front surface 13 up to an external load 16 to be
connected at the level of rear surface 14 of substrate 2. This
configuration enables the active surface of the microbattery to be
increased and also both enhances the electrochemical properties and
increases the integration density. This architecture does however present
a complex implementation in so far as several microfabrication steps
(deposition, etching, chemical mechanical polishing) are necessary to
perform the vertical electrical connections.

[0018] Other microbattery architectures with a vertical connection have
been proposed. The documents JP-A-61165965, US-A-2007087230 and
DE-A-102008011523 can be cited for example.

[0019] The solutions described in the prior art do however remain
insufficient to overcome the problems of mechanical stresses generated by
contact connection or they remain difficult to implement. In particular,
the presence of a localized area induces a fragile point within the
microbattery that is more particularly subject to volume deformations of
the microbattery, which is liable to crack, fissure or delaminate.

OBJECT OF THE INVENTION

[0020] The object of the invention is to provide a microbattery and the
manufacturing method of such a microbattery method remedying the
drawbacks of the prior art.

[0021] In particular, the object of the invention is to provide a
microbattery with good performances that presents an optimized energy
storage efficiency, a high integration density and an improved lifetime.
The microbattery is designed to be fabricated according to an industrial
manufacturing method that is easy to implement, is economically viable
and is compatible with technologies implemented in the microelectronics
field.

[0022] According to the invention, this object is achieved by a
microbattery and a method for manufacturing such a microbattery according
to the appended claims.

[0023] In particular, this object is achieved by the fact that the
electrical connections are formed by at least two electrically conductive
barriers passing through the encapsulation layer from the inner surface
to the outer surface of said encapsulation layer, each of said barriers
having a lower wall in direct contact with the front surface of the
second current collector/electrode assembly and an upper wall opening
onto the outer surface of the encapsulation layer, and by the fact that
said barriers form a compartmentalization network within the
encapsulation layer.

[0024] According to a particular development, at least two barriers are
adjacent and parallel to one another and are separated by a part of the
encapsulation layer.

[0025] According to a preferred embodiment, at least two barriers are
adjacent and parallel to one another and are separated by a part of the
encapsulation layer, and the distance separating the two adjacent and
parallel barriers is smaller than or equal to the inverse of the square
root of the dislocation density of said encapsulation layer.

BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Other advantages and features will become more clearly apparent
from the following description of particular embodiments of the invention
given for non-restrictive example purposes only and represented in the
appended drawings, in which:

[0027] FIG. 1 schematically represents a microbattery with a planar
electrical connection according to the prior art, in cross-section.

[0028] FIG. 2 schematically represents a microbattery with a vertical
electrical connection according to the prior art, in cross-section.

[0029] FIG. 3 schematically represents a microbattery according to a first
particular embodiment of the invention, in cross-section.

[0030] FIGS. 4 to 10 schematically represent different steps of a method
for manufacturing a microbattery according to FIG. 3, in cross-section.

[0031] FIG. 11 schematically represents a microbattery according to a
second particular embodiment of the invention, in cross-section.

[0032] FIG. 12 schematically represents a microbattery according to a
third particular embodiment of the invention, in top view.

[0033] FIG. 13 schematically represents a microbattery according to the
third particular embodiment of the invention, in cross-section along the
line AA of FIG. 12.

[0034] FIG. 14 schematically represents a microbattery according to a
fourth particular embodiment of the invention, in top view.

[0035] FIG. 15 schematically represents a microbattery according to the
fourth particular embodiment of the invention, in cross-section along the
line BB of FIG. 14.

DESCRIPTION OF PARTICULAR EMBODIMENTS

[0036] With reference to FIG. 3, a microbattery, also called "all-solid"
battery, comprises a stack 17 of solid thin layers formed on a substrate
2. Substrate 2 can be a silicon wafer or a silicon wafer containing an
Application Specific Integrated Circuit, or ASIC. Substrate 2 can also be
covered by a passivation layer (not shown) formed by a dielectric such as
silicon dioxide (SiO2) or by a bilayer successively formed by a
layer of SiO2 and a layer of silicon nitride (Si3N4). The
microbattery is preferably a lithium microbattery.

[0038] Stack 17 is covered by an encapsulation layer 18 and successively
comprises: [0039] a first current collector/electrode assembly 19 in
direct contact with substrate 2, [0040] a solid electrolyte 20, [0041] a
second current collector/electrode assembly 21 having a front surface 22
(at the top in FIG. 3) in direct contact with inner surface 23 of
encapsulation layer 18 and a rear surface 24 (at the bottom in FIG. 3) in
direct contact with solid electrolyte 20 and, [0042] electrical
connections 25 of second current collector/electrode assembly 21 to an
external electrical load.

[0043] The first and second current collector/electrode assembly,
respectively 19 and 21, are each formed by at least one thin layer.

[0044] According to a first particular embodiment represented in FIG. 3,
first current collector/electrode assembly 19 can be formed by a first
current collector 26 and a positive electrode 27. First current collector
26 is then in direct contact with substrate 2 and positive electrode 27
is located between first current collector 26 and solid electrolyte 20.

[0045] Second current collector/electrode assembly 21 can comprise a thin
layer forming a negative electrode 28 and a thin layer forming a second
current collector 29. Second assembly 21 is preferably formed by the
negative electrode 28 and the second current collector 29. As represented
in FIG. 3, second current collector 29 is formed on negative electrode 28
and solid electrolyte 20 is arranged between positive electrode 27 and
negative electrode 28. In this case, front surface 22 and rear surface 24
of second current collector/electrode assembly 21 are respectively formed
by the front surface of second current collector 29 and the rear surface
of negative electrode 28.

[0046] First and second current collectors, respectively 26 and 29, are
made from metal, for example from platinum (Pt), chromium, (Cr), gold
(Au), titanium (Ti), tungsten (W) or molybdenum (Mo).

[0047] Positive electrode 27 has a smaller dimension than that of first
current collector 26 so that positive electrode 27 does not extend beyond
the perimeter of first current collector 26 and is not in contact with
substrate 2 or with passivation layer of substrate 2. The materials used
as active materials of positive electrode 27 can be either non-lithiated
materials such as for example copper sulfides or disulfides (CuS or
CuS2), tungsten oxysulfides (WOySz), titanium disulfides
(TiS2), titanium oxysulfides (TiOxSy) or vanadium oxides
(VxOy) or lithiated materials such as for example lithium-based
mixed oxides such as lithium and cobalt oxide (LiCoO2), lithium and
nickel oxide (LiNiO2), lithium and manganese oxide
(LiMn2O4), lithium and vanadium pentoxide (LiV2O5) or
lithium and iron phosphate (LiFePO4).

[0048] Negative electrode 28 can conventionally be formed by one or more
materials chosen from transition metals and their alloys, metalloids and
their alloys, and lithium insertion or intercalation materials. Negative
electrode 28 can be a Li.sup.+ ion generator or also comprise an active
lithium insertion material. The materials used as active material of
negative electrode 28 can be non-lithiated materials such as for example
silicon (Si) and its alloys such as silicon/germanium alloy (SiGe), tin
and its alloys such as tin/copper alloy (Cu6Sn5), carbon or a
lithium insertion or intercalation material chosen from tin and silicon
oxynitrides (SiTON), tin nitrides (SnNx), indium nitrides
(InNx), and tin oxides such as tin dioxide (SnO2). The
materials used as active materials of negative electrode 28 can be
lithiated materials, for example a SiLix or GeLix alloy
containing less than 30% of lithium or a lithium-based mixed oxide such
as lithium and nickel oxide (LiNiO2).

[0049] The positive and negative electrodes, respectively 27 and 28, can
naturally have reversed positions in stack 17. Positive electrode 27 can
for example be at the top in FIG. 3 and negative electrode 28 be at the
bottom.

[0050] Solid electrolyte 20 is formed by a material enabling conduction of
Li.sup.+ lithium ions. Electrically insulating and ionic conductive
materials will preferably be chosen. Solid electrolyte 20 can be a
vitreous material having a base formed by boron oxide, lithium oxides or
lithium salts, or advantageously a lithium-based compound such as
phosphorus and lithium oxynitride (UPON) or a lithium phosphosilicate
oxynitride (LiSiPON).

[0051] Encapsulation layer 18 is conventionally formed by one or more
materials chosen from polymer, ceramic or metallic materials.

[0052] Electrical connections 25 are formed by at least one electrically
conductive barrier passing through encapsulation layer 18 from inner
surface 23 (at the bottom in FIG. 3) to outer surface 30 (at the top in
FIG. 3) of encapsulation layer 18. In other words, the barrier opens onto
outer surface 30 of encapsulation layer 18. What is meant by barrier is a
thin element of small thickness, determining separations in encapsulation
layer 18. The barrier has a lower wall 31, an upper wall 32 and side
walls 33 and preferably has a rectangular cross-section. The thickness of
the barrier is advantageously comprised between 0.5 μm and 10 μm,
preferably between 1 μm and 5 μm. The height of the barrier or of
side walls 33 of the barrier is at least equal to the thickness of
encapsulation layer 18. Lower wall 31 is in direct contact with front
surface 22 of second current collector/electrode assembly 21 and upper
wall 32 opening onto outer surface 30 of encapsulation layer 18.

[0053] The barrier is preferably formed by at least one electrically
conductive material, advantageously chosen from metals and their alloys,
polymers and metal oxides. The barrier is advantageously formed solely by
conductive materials, for example titanium, tungsten, platinum, nickel or
copper. The barrier is preferably solid, i.e. completely filled with
materials.

[0054] As represented in FIG. 3, upper wall 32 of the barrier preferably
forms a common flat surface with outer surface 30 of encapsulation layer
18. The barrier is thus not salient from the surface of encapsulation
layer 18.

[0055] Side walls 33 of each barrier (on the right and left in FIG. 3) are
further perpendicular to front surface 22 of second current
collector/electrode assembly 21. The volume expansion due to insertion
and extraction of lithium then has little effect on such an architecture
as the mechanical stresses are mainly generated in the same direction,
represented by the vertical arrow in FIG. 3, orthogonal with respect to
the main plane of substrate 2.

[0056] For an encapsulation layer 18 having a uniform thickness above
second current collector/electrode assembly 21, the height of the barrier
is equal in substance to the thickness of encapsulation layer 18 situated
above second assembly 21. What is meant by the height of the barrier is
the height of side walls 33 of the barrier.

[0057] For an encapsulation layer 18 having a non-uniform thickness of
layer above second current collector/electrode assembly 21, the height of
the barrier is in substance equal to the thickness of the part of
encapsulation layer 18 situated on each side of the barrier.

[0058] The presence of the barrier within encapsulation layer 18 enables a
discontinuity of the defects present in encapsulation layer 18 to be
created thereby reducing the critical diffusion paths of the species
liable to impair the active elements of microbattery stack 17. The
barrier blocks the impurities and creates a crack propagation stopper
area within encapsulation layer 18.

[0059] The barrier further having a small thickness, the contact section
between lower wall 31 and inner surface 23 of encapsulation layer 18 is
greatly reduced. The fragile area of the microbattery, that is devoid of
encapsulation and protected only by the barrier, is thereby greatly
reduced.

[0060] According to a particular embodiment, fabrication of such a
microbattery can be performed according to a manufacturing method that
comprises the successive steps represented in FIGS. 4 to 11 and described
in the following.

[0061] As represented in FIG. 4, the manufacturing method comprises
full-wafer deposition, on substrate 2, of a stack 17 of thin layers
successively forming a first current collector 26, a positive electrode
27, a solid electrolyte 20, a negative electrode 28 and a second current
collector 29. First current collector 26 and positive electrode 27 form
first current collector/electrode assembly 19 and second current
collector 29 and negative electrode 28 form second current
collector/electrode assembly 21.

[0062] This step is followed by full-wafer deposition of a first
protective layer 34 on second current collector/electrode assembly 21. In
this configuration, first protective layer 34 is deposited directly on
second current collector 29.

[0063] Full-wafer deposition is performed by any method, for example by
PVD or CVD. The thickness of the thin layers is typically comprised
between 0.1 μm and 5 μm.

[0064] First protective layer 34 is formed by one or more materials chosen
from polymer or ceramic materials that are advantageously non-conductive,
for example made from epoxide, acrylate, parylene, silica or nitride.

[0065] When a thermal annealing step is required to increase the
crystallization of the thin layer constituting one of the electrodes and
to increase its insertion capacity, this annealing step can be
advantageously formed after full-wafer deposition of the first thin layer
thus forming electrode 27.

[0066] As represented in FIG. 5, at least one trench 35 is then made in
first protective layer 34 by any method.

[0067] Trench 35 is conventionally made by conventional photolithography
masking processes followed by etching, for example, by selective dry
etching of Reactive Ion Etching (RIE) type or by selective wet process
etching.

[0068] Trench 35 forms a matrix to make a barrier. The barrier is designed
to introduce a discontinuity in first protective layer 34. What is meant
by matrix is a hollow imprint of the barrier. Trench 35 passes through
the thickness of first protective layer 34 and opens onto front surface
22 of second current collector/electrode assembly 21, i.e. the front
surface of second current collector 29.

[0069] One or more peripheral trenches 36 are also advantageously made
simultaneously with trench 35 in order to define the final shape and
dimensions of the microbattery. Peripheral trenches 36 also pass through
the thickness of first protective layer 34 and open onto front surface 22
of second current collector/electrode assembly 21, i.e. the front surface
of second current collector 29.

[0070] As represented in FIGS. 6 and 7, trench 35 is then filled with an
electrically conductive material. This filling step comprises deposition
of a thin layer 37 of electrically conductive material on first
protective layer 34 by any known method, followed by a selective etching
of thin layer 37. For example purposes, the selective etching can be a
dry process etching.

[0071] As represented in FIG. 6, the thickness of trench 35, noted
X1, is preferably smaller than or equal to twice the thickness of
thin layer 37, noted X2, so that trench 35 is completely filled by
the electrically conductive material forming thin layer 37.

[0072] The thickness of peripheral trench or trenches 36, noted X3,
is preferably larger than or equal to twice the thickness X2 of thin
layer 37. Peripheral trenches 36 delineating the microbattery are thus
not completely filled.

[0073] As represented in FIG. 7, thin layer 37 is etched by any known
method, for example by dry process etching. On account of the particular
dimensioning of trenches 35 and 36, thin layer 37 located in peripheral
trenches 36 and on outer surface 30 of first protective layer 34 (at the
top in FIG. 7) is eliminated as the parts of thin layer 37 are directly
exposed to the etching solution whereas the part of thin layer 37 in
trench 35, which is narrower, is not. On completion of this etching step,
only the part of thin layer 37 in trench 35 remains.

[0074] This step enables a barrier to be obtained within first protective
layer 34 and furthermore does not require a masking operation prior to
etching.

[0075] As represented in FIG. 8, after trench 35 has been filled, one or
more etching steps of stack 17 are performed, by any known method, to
localize the microbattery on substrate 2. The type of etching used
depends on the nature of the thin layers to be etched. It can thus be
envisaged to perform masking by photolithography followed by a single
etching operation to simultaneously etch second current collector 29,
negative electrode 28, solid electrolyte 20 and positive electrode 27. A
part of the surface of first protective layer 34 is then exposed (on the
right and on the left in FIG. 8). This etching step gives the final shape
of active stack 17 of the microbattery. The exposed part of first
protective layer 34 can be located around stack 17.

[0076] According to an alternative embodiment, etching of stack 17 is
performed by a succession of selective etchings for one or more thin
layers corresponding to second current collector 29, to negative
electrode 28, to solid electrolyte 20 or to positive electrode 27.

[0077] As represented in FIG. 9, a second protective layer 38 is made by
any known method, for example by deposition followed by etching. Second
protective layer 38 forms a continuous uninterrupted layer with second
protective layer 38. Second protective layer 38 is conventionally formed
by one or more materials chosen from polymer, ceramic or metallic
materials. First and second protective layers, respectively 34 and 38,
are advantageously formed by the same material or materials. Second
protective layer 38 covers all the lateral parts of stack 17 (on the
right and on the left in FIG. 9) and extends on each side of stack 17 on
first current collector 26, without however covering all of the exposed
part of first current collector 26. First and second protective layers,
respectively 34 and 38, thus form encapsulation layer 18.

[0078] The exposed surface of first current collector 26, situated at the
periphery of the microbattery, constitutes the contact points for
connection to an external electrical load. Thus, as illustrated in FIG.
10, contact pads 39 enable an external electrical load (not shown) to be
connected to first and second current collectors, respectively 26 and 29,
and consequently to positive electrode 27 and negative electrode 28 of
the microbattery. Contact pads 39 can be arranged on each side of stack
17, on the exposed surface of first current collector 26. Contact wires
are conventionally soldered onto contact pads 39 and connected to
external electrical load (FIG. 10).

[0079] According to a second particular embodiment represented in FIG. 11,
the microbattery is a "Li free" microbattery. Before the first charging
of the micro-battery, second current collector/electrode assembly 21 is
initially formed by a single thin layer. The latter is formed by a metal
or a metal alloy. This thin layer constitutes a plating layer for
electrolytic deposition of metallic lithium. The lithium deposition is
then activated when charging of the micro-battery is performed. As
described in the document WO-A1-0060689, before its initial charging, the
microbattery does not comprise a thin layer of metallic lithium
constituting negative electrode 28. The latter is formed by the
electrolytic deposition of metallic lithium when charging of the
microbattery takes place. After the first charging of the microbattery,
second current collector/electrode assembly 21 is formed by a thin layer
constituting second current collector 29 and a thin layer of metallic
lithium constituting negative electrode 28 (FIG. 10).

[0080] The method for manufacturing the microbattery according to the
second particular embodiment is identical to the first embodiment
described in the foregoing, with the exception that negative electrode 28
and second current collector 29 are achieved by deposition of a single
thin layer.

[0081] According to a third particular embodiment represented in FIGS. 12
and 13, electrical connections 25 are formed by at least two electrically
conductive barriers. The barriers form a compartmentalization network 40
within encapsulation layer 18.

[0082] What is meant by "compartmentalization network" is an assembly
formed by at least two compartmenting barriers that cross one another,
advantageously in regular manner, said barriers dividing encapsulation
layer 18 into several compartments. The compartmentalization network
thereby creates one or more crack propagation area(s) 41 that are liable
to form within encapsulation layer 18, during operation of the
microbattery.

[0083] Each of the barriers has a lower wall 31 in direct contact with
front surface 22 of second current collector/electrode assembly 21 and an
upper wall 32 opening onto outer surface 30 of encapsulation layer 18.

[0084] According to an advantageous configuration, at least two barriers
are adjacent and parallel to one another. The two barriers are thus
separated from one another by a part of the encapsulation layer 18. Side
walls 33 of the barriers are perpendicular to front surface 22 of second
current collector/electrode assembly 21 and parallel to one another (FIG.
13).

[0085] The distance separating the two adjacent and parallel barriers,
noted L1, is preferably smaller than or equal to the inverse of the
square root of the dislocation density of encapsulation layer 18, noted
dd. What is meant by dislocation density of encapsulation layer 18
is the density of defects per surface unit. Thus, to reduce or eliminate
the defect discontinuity, the configuration of the microbattery, in
particular the arrangement of the barriers with respect to one another,
has to comply with the following relation:

L 1 ≦ 1 d d ##EQU00001##

[0086] For example, for a dislocation density of 4 disloc/μm2,
distance L1 separating the two adjacent and parallel barriers has to
be smaller than or equal to 0.5 μm2.

[0087] Compartmentalization network 40 creates a discontinuity of the
defects present in encapsulation layer 18, thereby reducing the critical
diffusion paths of the species liable to impair the active elements of
stack 17 of the microbattery. Likewise, each barrier of
compartmentalization network 40 creates a crack propagation stopper area
41 within the encapsulation layer. The architecture of the microbattery
thus limits the fragile areas of the microbattery, increases the
integration density and has the effect of improving the performances of
the microbattery.

[0088] According to a fourth particular embodiment represented in FIGS. 14
and 15, compartmentalization network 40 has a meshed structure with
patterns in the form of a parallelogram, for example with a square or
rectangular pattern.

[0089] As illustrated in FIG. 14, compartmentalization network 40
preferably has a structure in the form of a grid. The meshing of the
barriers forms compartments isolated from one another in encapsulation
layer 18 preventing any diffusion of species liable to impair the
elements constituting stack 17 and cracks 41.

[0090] Upper walls 32 of the barriers further form connection lines at the
surface of encapsulation layer 18 enabling electrical connection of the
microbattery to one or more external electrical loads. As illustrated in
FIG. 14, at least one contact pad 39 can for example be placed on a
connection line in contact with one of upper walls 32 of
compartmentalization network 40 and connected to the negative terminal of
an external electrical load (not shown). At least one other contact pad
39 is then placed on the exposed surface of first current collector 26
and connected to the positive terminal of the external electrical load.

[0091] According to an alternative embodiment that is not represented, the
exposed surface of first current collector 26 is situated on one side of
encapsulated stack 17 only, in order to limit the surface of the
substrate dedicated to the current collector and to improve the
integration density.

[0092] The method for manufacturing a microbattery according to the third
and fourth embodiments is identical to that of the first embodiment
described in the foregoing with the exception of the form of the matrix
enabling electrical connections 25 to be achieved. The method thus
comprises the formation in first protective layer 34 of at least one
trench 35 which forms the matrix to achieve the barriers. The barriers
can conventionally be made from a single trench or from several trenches.

[0093] The invention is in no way limited to the embodiments described in
the foregoing. In particular, encapsulation layer 18 is formed solely by
first protective layer 34. The manufacturing method then differs from
that of the first embodiment described in the foregoing in that the
etching of the stack 17 is performed before that of first protective
layer 34 and in that first protective layer 34 is deposited on first
current collector 26 and stack 17 so that it completely covers stack 17.
The trench or trenches are then made directly on encapsulation layer 18.

[0094] A lithium microbattery having an optimized architecture, with
vertical connection, has been achieved according to the manufacturing
method of the invention. The architecture of this battery avoids the
presence of contact connection which fragilizes encapsulation layer 18
and impairs its mechanical and hermetic properties.

[0095] The microbattery according to the invention has an architecture
that enables the surface of the substrate dedicated to the current
collectors to be limited and the integration density to be improved. The
presence of barriers, and in particular of a compartmentalization
network, further enables the barrier effect of the encapsulation layer to
be preserved limiting the fragile areas and the performances of the
battery to be improved, in particular the lifetime, comparatively with
microbatteries of the prior art.

[0096] These microbatteries find numerous industrial applications in
particular in the field of microelectronics where miniaturization of
components and autonomy requirements impose the use of smaller, more
powerful storage batteries having longer lifetimes.

[0097] Unlike manufacturing methods according to the prior art which
require several often complex steps of deposition and etching of the
different layers forming the lithium microbattery, the manufacturing
method according to the invention is easy to implement and simple. In
characteristic manner, the manufacturing method can be performed with a
reduced number of etching and masking steps, in particular a single step
being necessary for the etching of stack 17. The functional interfaces of
the microbattery being protected in the course of the manufacturing
method, the quality of these interfaces is further preserved.

[0098] Such a manufacturing method of a lithium microbattery is compatible
with technologies implemented in the microelectronics field, which in
particular enables such a lithium microbattery to be incorporated on a
microcomponent and results in manufacturing costs being able to be
reduced.